If you’re excited to see Marvel Studios’ Iron Man 3 this weekend, you’re not alone; early estimates suggest that it might have moviegoers even more excited than they were for last year’s Marvel’s The Avengers. That might seem counter-intuitive: Wouldn’t people be more excited about seeing a bunch of their favorite heroes joining forces,than the third installment of Tony Stark’s solo adventures? There is, however, a very good reason for this: Iron Man is currently the only Marvel movie superhero who is really worth watching on screen — and throwing the other heroes in a movie with him just dilutes his appeal.

Even the most ardent fan of either Captain America: The First Avenger or Thor has to admit that those movies were superficially stirring and oddly hollow. On paper, they seemed to have everything audiences expect from a superhero summer movie — High stakes! Action! Romance! (But not too much of the latter, because, you know, superheroes!) — but in actual execution, there was something missing. You found yourself leaving the movie theater exhilarated, only to have trouble remembering later what it was that you were quite so excited about.

It takes about two minutes of watching Robert Downey Jr. in action in Iron Man 3 — in any of his appearances as the armored Tony Stark, in fact — to realize what the other Avengers are lacking: Charisma. Downey Jr. is an irresistible presence on screen, and Tony Stark — a character simultaneously cocky and vulnerable, an only slightly repentant jerk who wins you over despite himself — is a role he plays with visible relish; when he’s on screen, you can forgive almost anything (see: Iron Man 2 and the logic-bending nonsense contained therein) purely because Downey Jr.’s Stark is just so damn cool.

By contrast, you have Chris Hemsworth’s Thor and Chris Evans’ Captain America, two men who could hardly seem more wooden and emotionally uninvested. Hemsworth’s Thor seems the complete opposite of Downey Jr.’s Iron Man, sucking the air out of every scene with every glower and deep-voiced murmur of dialogue. Similarly, Evans’ performance as the star-spangled Avenger looks as if he misread “upright hero” as “uptight hero” in his script, and it stuck. I doubt that the problem is with the actors, though; watch Evans in another comic book movie, the severely-underrated Scott Pilgrim Versus The World, and you’ll see an actor who is easily as much fun to watch as Robert Downey Jr.

I suspect the fault lies in some unfortunate conception of what we want from our blockbuster movies, and the unfortunate effect that has on characterization as a whole, and characterization of our heroes in particular. In recent years, it’s become increasingly obvious that story comes a distant second to spectacle in modern summer blockbusters. As long as the audience can be wowed into submission with CGI characters and worlds that could never exist in reality, it seems, plot holes or a paucity of internal logic can be forgiven or at least excused, if the success of Michael Bay’s Transformers movies is anything to go by. With this mindset gradually becoming common wisdom, writing has become less about telling an engaging story, and more a matter of solving “How do we get from Set Piece A to Set Piece B in the least number of moves?”

As a result, characters have devolved from individuals into interchangeable “types” acting as just stand-ins and props for us to watch run towards or away from the next big action scene. That’s fine for those who have the fun “types” to play with: the cowards, the jerks, and others tragically flawed. But for the heroes of the piece, anything resembling a quirk or personality gets ironed out in favor of minimizing the need for exposition, which is (of course) focused on the next big special-effects scene.

For Iron Man, this works out fine: Robert Downey Jr. gets to play a jerk who (deep down) means well, and does it both wonderfully and winningly. For Captain America and Thor, however — characters who border on the cartoonishly perfect even in the hangdog world of Marvel Comics — it means that Chrises Evans and Hemsworth are left with nothing to do aside from stand around looking handsome and let everyone else have the fun.

That works in Avengers —Downey Jr. and Mark Ruffalo’s Bruce Banner can play off the straight men, after all —- but in their own movies…? How long can underwritten heroes really hold our attentions, no matter how elaborate the visuals? Perhaps we should enjoy Iron Man 3 as much as we can. It could be all downhill from here.